1. Field
This invention pertains to radiolocation of transmitter-equipped stations within the coverage of a source of precisely timed radio signals such as Omega.
2. Prior Art
The long range VLF (very low frequency) navigation system known as Omega has been operational for some time, and is presently being extended to cover the entire globe. Various systems such as differential Omega have been devised to provide relative positional information of improved accuracy in the generally localized area of a reference station that receives the VLF Omega signals both directly and as relayed or rebroadcast from a target or slave station, usually at some higher frequency and without change in format. The direct and relayed transmissions are processed at the reference station to determine the position of the target or slave station with respect to the reference station.
U.S. Pat. No. 3,680,115 describes such a system. The number of relays that can be operated simultaneously is limited by the available frequency spectrum, because each target or slave station requires its own separate relay channel. Moreover, at least three Omega stations are required to obtain an unambiguous fix.
The internationally standardized Air Traffic Control Radio Beacon System (ATCRBS) uses the same carrier frequencies and signal formats throughout the world for cooperation between Secondary Surveillance Radars (SSRs) and airborne transponders. The primary purpose of ATCRBS is to provide ground-based positional information regarding aircraft within the service area of an SSR, for use by traffic controllers.
Transponders are intended to transmit replies only in response to received SSR interrogations. Since a transponder receives interrogations only when a narrow rotating radar beam is pointing at it, it remains silent most of the time, even in a dense radar environment. Occasionally a transponder emits a reply message without interrogation. This is called "squitter" and has little or no effect on the system operation because the SSR sees it as background noise. There is an established maximum permissible squitter rate, much higher than that of the usual transponder.